Afterburner flameholder

ABSTRACT

An afterburner flameholder for an aircraft gas turbine engine is provided with a continuous annular inlet together with a closely adjacent inlet screen to meter gas flow from the turbine exhaust to a plurality of discrete pilot fuel jets within the flameholder. The wake behind the inlet screen is highly turbulent and operates to minimize temperature gradients existing in the inlet flow. The annular inlet communicates with smooth and uniformly flared flow passages through the flameholder thereby inhibiting local variations in flow velocity which heretofore were caused by sudden disruptions in the flowpath.

1 ttes Hufnagel et a].

[ Get. 16, 11973 AFTERBURNER FLAMEHOLDER [75] Inventors: Robert Hughes Hufnagel,

Marblehead; Lowell Jackson Pierce,

Melrose; Edward Joseph Sabol, Danvers, all of Mass.

[73] Assignee: General Electric Company, Lynn,

Mass.

[22] Filed: Sept. 8, 1972 [21] Appl. No.: 287,281

[52] US. Cl 60/261, 60/39.32, 60/39.72 R, 69/3974 R [51] Int. Cl F02c 7/20, F02c 7/22 [58] Field of Search 60/39.72 R, 39.74 R, 60/261, 39.32

[56] References Cited I UNITED STATES PATENTS 2,296,023 9/1942 Dallenbach et al. 60/39.72.R 2,679,137 5/1954 Probert 60/261 X 3,054,259 9/1962 Arthur 60/39.72 R 3,355,885 12/1967 Toone 60/39.72 R X 3,156,094 11/1964 Nash et a1. 60/39.72 R X 7 Primary Examiner-Carlton R. Croyle Assistant Examiner--Robert E. Garrett Attorney-Edward S. Roman et al.

[5 7] ABSTRACT An afterburner flameholder for an aircraft gas turbine engine is provided with a continuous annular inlet together with a closely adjacent inlet screen to meter gas flow from the turbine exhaust to a plurality of discrete pilot fuel jets within the flameholder. The wake behind the inlet screen is highly turbulent and operates to minimize temperature gradients existing in the inlet flow. The annular inlet communicates with smooth and uniformly flared flow passages through the flameholder thereby inhibiting local variations in flow velocity which heretofore were caused by sudden disruptions in the fiowpath.

8 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION In general, this invention relates to an afterburner flameholder and, more particularly, to an afterburner flame-holder for inclusion within the gas stream of an aircraft gas turbine engine as part of an afterburning system to provide additional thrust augmentation.

It is well known in the aircraft gas turbine art to provide thrust augmentation by burning additional fuel in an afterburner located downstream of the engine turbine. The afterburner generally includes a means for dispersing a main flow of fuel together with a flameholder to which the flame may attach. The flameholder reduces locally the velocity of the gas stream in order to sustain the flame which would otherwise blow out. One well known type of flameholder comprises two concentric flame rings arranged to diverge from each other in a downstream direction. The main flow of fuel is dispersed in a manner that allows fuel droplets to impinge upon the outside diverging surfaces of the flameholder wherein the afterburning flame attaches to the trailing edges of the flame rings.

In order to provide for positive and uniform lightoff of the afterburner during all modes of flight operation, pilot fuel may be introduced and sparked to ignite by means of a point source igniter. The pilot flame in turn operates to ignite the main fuel droplets. It is well known to introduce the pilot fuel to the afterburner by means of discrete jets situated around the flameholder. The pilot fuel jets are generally located intermediate the flame rings suchthat each pilotjet receives gas flow from the turbine exhaust through an individual inlet opening in the leading edge of the flameholder.

The disadvantages to this arrangement are twofold. First, the temperature of the gas flow into the flameholder generally varies circumferentially around the flameholder. These circumferential temperature gradients are preserved by the conventional system of individual inlet openings for providing gas flow to the pilot fuel jetsand can lead to localized overheating of the flameholder precipitating a condition commonly referred to as hot streaking" wherein the flameholder rapidly deteriorates in the overheated areas.

The other disadvantage to conventional afterburner flame-holders relates to their susceptibility to phenomena commonly referred to as flashback" and preignition." Flashback can be generally described as an upstream propagation of the afterburner flamefront into the interior of the flameholder in the area between the flame rings, and may occur if the velocity of the flow through the flameholder falls off locally below the minimum velocity required to maintain flame attachment. Preignition is not as well understood as flashback, and relates to a sudden spontaneous ignition of the flow in the area between the flame rings of the flameholder. Preignition differs essentially from flashback in that the upstream preignition flame inside the flameholder exists independently of the downstream flame which remains attached to the flame ring trailing edges. Preignition may be a direct result of temperature gradients in the flow through the flameholder. The deleterious effects from flashback and preignition are essentially the same resulting in a premature localized burning of the flameholder components.

Flashback and preignition may also result from local velocityvariations of the flow through the flameholder. Such non-uniform velocity profiles can result from abrupt area changes or discontinuities in the flowpath through the flameholder, and are common to conventional flameholders particularly those employing the system of individual inlet openings serving each pilot fuel jet.

Therefore it is an object of this invention to provide an afterburner flameholder, together with a system of pilot fueljets wherein the gas flow inlet to the pilot fuel jets is designed to minimize temperature gradients in the gas flow through the flameholder.

It is also an object of this invention to provide a flameholder wherein the flowpath therethrough is smooth and uniformly flared without abrupt changes or discontinuities thereby reducing the risk of either flashback or preignition occurring.

SUMMARY OF THE INVENTION A gas turbine engine of the type having a compressor, combustor, and turbine in serial flow relation is provided with thrust augmentation by an afterburner. The afterburner includes a flameholder having an inner flame ring,- and an outer flame ring spaced radially outward and concentric to the inner flame ring. Means are further included for introducing a main flow of fuel outside the flame rings together with means for attaching the flame rings to the engine casing. According to the means of this invention, the outer ring in cooperation with the inner ring defines an annular inlet, for receipt of hot gas flow from the turbine, together with an annular outlet. An inlet screen is provided between the inner and outer flame rings closely adjacent the annular inlet for metering the flow between the flame rings and precipitating turbulence in the wake behind the screen. The high turbulence of the wake behind the screen operates to minimize any circumferential temperature gradients in the inflowing gas. Means are included for introducing pilot fuel in discrete jets downstream of the inlet screen. Circumferentially spaced apart airfoil type swirl vanes, radially extending between the inner and outer flame rings, are provided downstream of the pilot fuel injection means together with means for igniting the pilot fuel. The gas-fuel mixture is swirled in a circumferential direction by the swirl vanes in order to provide for a rapid propagation of flame around the entire flameholder.

DESCRIPTION OF THE DRAWINGS While the specification concludes with claims distinctly claiming and particularly pointing out the invention described herein, it is believed that the invention will be more readily understood by reference to the discussion below and the accompanying drawings in which:

FIG. 1 is a side elevational view partly in crosssection of a gas turbine engine of the turbofan type embodying the afterburner flameholder of this invention.

FIG. 2 is an enlarged cross-sectional view of the afterburner flameholder of FIG. 1.

FIG. 3 is a cross-sectional view taken along the line 33 of FIG. 2.

FIG. 4 is an alternate embodiment for the crosssectional view taken along the line 33 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is seen a turbofan engine comprising a core engine 12 which generates a hot gas stream for driving a fan turbine 14. The turbine 14 is connected to and drives the rotor 15 of a fan 16 disposed at the inlet end 17 of the engine. The core engine 12 qnd the fan turbine 14 are disposed within an inner casing 18. An elongated cowl, or outer casing 20 defines the engine inlet indicated at 17 and, in combination with the casing 18, defines a duct 22, concentric of the core engine 12.

In operation, the fan 16 pressurizes an air stream, the outer portion of which passes along the duct 22 and the inner portion of which enters the core engine 12. In the core engine, the air stream is further compressed by a core engine compressor 24 to provide a highly pressurized air stream for supporting combustion of fuel in a combustor 26. Fuel to the combustor 26 is provided by fuel injection means 27 which receives a flow of pressurized fuel through conduit 28 from a source of pressurized fuel (not shown). The hot gas stream thus generated drives a high pressure, core engine turbine 30 which is connected to the rotor of the compressor 24. The hot gas stream passes through the fan turbine 14 and then it is intermixed with a portion of the bypass air from duct 22 entering through an annular passageway 32.

Additional thrust augmentation may be provided by the afterburner shown generally at 33. Additional fuel is introduced to the afterburner 33 through fuel inlet means 34 connecting to a source of pressurized fuel (not shown). Means for afterburner flame attachment are provided by flameholder 36, the details of which will be made fully obvious from the discussion below.

The hot gas stream exiting from the afterburner 33 is discharged from a nozzle 38 to provide thrust for propulsion of an aircraft. Although the invention is described in relation to a turbofan engine, it could be applied with equal success to other types of gas turbine engines such as the straight turbojet engine.

Looking now to FIG. 2, where like numerals refer to previously described elements, there is seen the flameholder 36 comprising an inner frusto conical flame ring 40 and an outer frusto conical flame ring 42. The inner and outer flame rings 40, 42 are arranged in general concentric alignment so as to define an annular inlet 44 and an annular outlet 46 of greater cross-sectional area than the inlet. Radial separation between the flame rings is maintained by a plurality of circumferentially spaced apart and radially extending swirl vanes 48, the profiles of which may be best seen in FIG. 3.

The flameholder ring 36 is attached to the inner casing 18 by means of a plurality of circumferentially spaced apart retaining links 50. The outer radial ends of the retaining links 50 are rotatably pinned at pivot 52 to flange elements 54 which are in fixed connection between the inner casing 18 and the outer casing 20. The inner radial end of each retaining link 50 is rotatably pinned at pivot 55 to clevis arrangement 56 extending radially outward ofa piston 58. Piston 58 is disposed within a sleeve 60 which extends through the inner and outer flame rings 40, 42 in generally central alignment with one of the swirl vanes 48. In order to accommodate rotation of the retaining link 50 about the pivot 55, a notch 62 must be provided through the outer flame ring 42, and also the outer radial end 64 of sleeve 60 must be skewed relative to the longitudinal axis of the sleeve. The pivotal feature of the retaining links 50 is provided to accommodate thermal expansion of the flame rings 40, 42 which may result in a radially outward growth of the flameholder 36. The number of retaining links 50 may be varied to suit particular hardware requirements with a minimial requirement of at least three retaining links spaced apart at equal intervals.

Adjacent the flameholder annular inlet 44, there is provided an inlet screen 68 preferably comprising a plurality of circumferentially spaced apart and radially extending hollow cylinders 70. The inlet screen 68 meters a pre-determined portion of the inlet gas flow through the confined passages of the flameholder 36. Although other types of inlet screens would be well within the scope of the invention, spaced apart hollow cylinders 70 are preferred because the metered flow through the screen remains relatively insensitive to fluctuations in direction of the inlet gas flow. The hollow cylinders of the inlet screen 68 are also convenient to manufacture and easily accommodate to outward radial thermal growth.

The afterburner fuel inlet means 34 comprises a plurality of main fuel inlet conduits 74, some of which extend radially through the flameholder 36. Each main fuel inlet conduit 74 includes an aperture 78 closely adjacent the end thereof so as to discharge main jet streams of fuel about the outer surfaces of the flameholder 36. The apertures 78 are spaced radially apart and oriented so as to discharge main jet streams of fuel in directions generally normal to the gas stream. A collar 72 is provided in spaced apart relation around the main fuel inlet conduits 74 to permit outward radial thermal expansion of the flameholder 36 without interference from the main fuel inlet conduits.

A pilot fuel conduit 76 terminates between the inner and outer flame rings 40, 42. Each pilot fuel conduit 76 includes two circumferentially spaced apart apertures 77, 77', which may be best seen in FIGS. 3 and 4. The apertures 77, 77' are arranged to direct two divergent jet streams of pilot fuel, each stream of which flows through the nozzle defined by two adjacent swirl vanes 48. The pilot fuel conduits 76 need only be located adjacent the leading edges of every other swirl vane 48 in order to provide ajet stream of pilot fuel to each nozzle. lgniter 80 provides a point source of spark to ignite the pilot fuel.

Referring now to FIG. 3, there are shown the swirl vanes 48 which generally comprise a frusto conical main body portion 82. An integral tail portion 84 extends rearwardly and circumferentially from the main body portion 82. Side walls 86, 88 of the integral tail portion 84 converge rearwardly from tangential intersection with the frusto conical main body 82 and intersect at a swirl vane trailing edge 90. The throat width A of each nozzle between adjacent swirl vanes 48 is selected to achieve the minimum gas flow velocity therethrough that will maintain flame attachment to the trailing edges 90.

Describing now the operation of the foregoing structure for cases where additional thrust is required, the afterburner may be lit by first introducing pilot fuel through conduit 76, whereupon the pilot fuel is sparked to ignite by igniter 80. Inlet screen 68 meters a predetermined portion of the total gas flow through the annular inlet 44 so as to achieve the desired stoichiometric ratio of fuel to air. The hollow cylinders or rods 70 provide for a highly predictable and uniform metered flow through theflameholder.

As previously discussed, the temperature of the gas flow into the flameholder 36 may vary circumferentially around the flameholder. Such circumferential temperature gradients can precipitate hot streaks within the flameholder leading to rapid localized deterioration of the flameholder. The wake behind the inlet screen 68, however, provides a high level of turbulence so as to thoroughly mix the incoming gas flow, thereby minimizing the circumferential temperature gradients which can lead to the premature deterioration of the flameholder. Therefore, the continuous annular inlet feature of the flameholder of this invention coupled with the inlet screen provides a marked advantage over conventional flameholders having individual discrete inlet openings servicing each pilot fuel jet.

The turbulent wake behind the inlet screen 68 also provides for a high degree of mixing and dispersing of pilot fuel within the hot incoming gas. This highly uniform gas-fuel mixture flows downstream between the swirl vanes 48 to the flame zone which is desirably maintained behind the swirl vane trailing edges 90. The gas-fuel mixture is swirledin a circumferential direction in order to provide for a rapid propagation of flame around the entire flameholder from the initial point of spark at igniter 80. Main fuel is ignited by the annular pilot flame in a conventional manner.

One problem previously encountered with afterburner flameholders relates to the previously discussed phenomenon called flashback, which is an upstream propagation of the afterburner flame front into the interior passages of the flameholder. Flashback may occur if the velocity of the fuel-gas mixture between the swirl vanes 48 falls off below the minimum speed required to maintain flame attachment at the swirl vane trailing edges 90. Another phenomenon previously discussed is preignition which is'not as well understood as flashback and relates to a sudden spontaneous ignition of the fuel-gas mixture in an area upstream of the swirl vane trailing edges 90. Preignition differs essentially from flashback in that the upstream preignition flame inside the flameholder is separate and distinct, existing independently of the downstream flame attached to the swirl vane trailing edges 90. However, the deleterious effects from flashback and preignition are essentially the same resulting in a premature localized burning of the flameholder components. Preignition may be a direct result of the circumferential temperature gradients previously discussed. Therefore, the probability of preignition occurring in the flameholder of this invention is believed to be minimized by the thorough mixing effect of the highly turbulent wake behind the inlet screen 68.

Local variations in velocity and flow direction may also contribute to preignition and flashback. Therefore, it is desirable that the velocity profile be kept uniform and predictable over all cross-sections taken between the plane of pilot fuel injection at inlet conduit 76 and the plane defined by the swirl vane trailing edges 90. However, if the flow paths through the afterburner are designed with abrupt area changes or discontinuities as has been the case for conventional afterburner flameholders, then local perturbations and eddies will develop and disrupt the uniform nature of the velocity profiles. These preturbations and eddies will cause the velocity of the gas-fuel mixture approaching the swirl vane trailing edges to fall off locally below the minimum velocity required to maintain flame attachment at the trailing edges. In such cases the afterburner flamefront will propagate upstream into the interior passages of the flameholder at those areas of flow velocity disruption causing localized overheating and burning of the flameholder components.

The flow passages of the flameholder of this invention, however, have been specifically designed to be smooth and uniformly flared with no sudden area changes or discontinuities in the passageways between the plane of pilot fuel injection at inlet conduit 76 and the plane defined by the swirl vane trailing edges 90. The high turbulence of the wake behind the inlet screen 68 becomes substantially exhausted before reaching the swirl vane trailing edges 90 so as to provide a substantially uniform velocity profile at the trailing edges 90. Swirl vanes 48 are essentially airfoil shaped with gradually contoured sides presenting no sharp discontinuities to the fuel-gas flow therethro'ugh which could precipitate localized flashback or preignition. As is readily obvious, the geometric configuration is simple and provides for a readily predictable flow between the swirl vanes. The configuration also lends itself for economical low cost tooling and manufacture.

Referring now to FIG. 4, there is shown a slight modiflcation of the swirl vanes 48. A sharp integral leading edge portion 92 has been included forward of the main body portion 82 and includes sidewalls 96, 98 converging forward from tangential intersection with the main body 82, thereby streamlining the aerodynamic profile of each swirl vane 48. Also sidewall 86 of the integral tail portion has been extended rearwardly and circumferentially at 94 beyond the line of intersection 90 of the converging sidewalls 86, 88. The angle of discharge of the gas-fuel mixture from swirl vanes 48 may therefore be made to more nearly approximate the circumferential direction, thereby reducing the overall time required for flame propagation around the annular flameholder. Also, convective cooling of the vane trailing edges is further facilitated by this arrangement.

Various changes could be made in the structures shown in FlGS. 1 through 4 without departing from the scope of the invention. The swirl vanes 48 may be of any suitable airfoil cross-section so long as a prescribed and predictable flow can be achieved between the vanes. The inlet screen 68 could alternatively be a reticulate configuration. Therefore, having described preferred embodiments of the invention, though not exhaustive of all possible equivalents, what is desired to be secured by letters patent is claimed below.

What is claimed is:

1. In a gas turbine engine of the type having a casing enveloping a compressor, combustor, and turbine in serial flow relation, augmentation is provided by an afterburner comprising:

an inner flame ring;

an outer flame ring, spaced radially outward and concentric to the inner flame ring, which in cooperation with the inner flame ring defines an annular inlet for receipt of hot gas flow from the turbine, together with an annular outlet;

an inlet screen between the inner and outer flame rings closely adjacent the annular inlet for metering the flow between the flame rings wherein the turbulence of the wake behind the screen minimizes circumferential temperature gradients of the gas inflow;

means for introducing pilot fuel in discrete jets downstream of the inlet screen;

circumferentially spaced apart airfoil type swirl vanes radially extending between the inner and outer flame rings downstream of the pilot fuel means; means for igniting the pilot fuel;

means for introducing a main flow of fuel outside the flame rings, and

means for maintaining the flame rings in fixed relation relative to the engine casing.

2. The afterburner of claim 1 wherein the attaching means comprise a plurality of retaining links pivotally connected at one end to the engine casing; and

a plurality of pistons radially disposed in connection to the flame rings and pivotally connected to the other ends of the retaining links, thereby accommodating thermal expansion of the flame rings.

3. The afterburner of claim 1 wherein:

the inner and outer flame rings are conical frustums cooperatively arranged to define an annular rearwardly diverging passageway.

4. The afterburner of claim 1 wherein:

the inlet screen comprises a plurality of circumferentially spaced cylinders extending radially between the inner and outer flame rings.

5. The afterburner of claim 1 wherein each swirl vane comprises a generally frusto conical main body portion with an integral tail portion extending rearwardly and circumferentially from the main body portion with the sidewalls of the integral tail portion converging rearwardly from tangential intersection with the frusto conical main body into intersection at the swirl vane trailing edge.

6. The afterburner of claim 5 wherein each swirl vane includes a sharp integral leading edge portion extending forward of the main body portion with the sidewalls of the integral leading edge portion converging forwardly from tangential intersection with the frusto conical main body into intersection at the swirl vane leading edge and wherein:

a sidewall of the integral tail portion extends rearwardly and circumferentially beyond the line of intersection of the converging tail portion sidewalls.

7. The afterburner of claim 1 wherein:

the pilot fuel means includes an inlet conduit terminating between the flame rings at a point closely adjacent the leading edge of a swirl vane and having two spaced apart apertures for directing individual jets of pilot fuel on each side of the adjacent swirl vane, and wherein:

the main fuel means includes a plurality of inlet conduits terminating radially outward and inward of both flame rings, each conduit ofwhich includes an aperture for directing ajet of main fuel outside the flame rings.

8. A flameholder comprising:

an inner flame ring;

an outer flame ring spaced radially outward and concentric to the inner flame ring which in cooperation with the inner flame ring defines an annular inlet and outlet;

an inlet screen between the inner and outer flame rings closely adjacent the annular inlet for metering the flow between the flame rings wherein the turbulence of the wake behind the screen minimizes circumferential temperature gradients of the inflow;

means for introducing pilot fuel in discrete jets downstream of the inlet screen;

circumferentially spaced apart airfoil swirl vanes radially extending between the inner and outer flame rings downstream of the pilot fuel means; and

means for igniting the pilot fuel. 

1. In a gas turbine engine of the type having a casing enveloping a compressor, combustor, and turbine in serial flow relation, augmentation is provided by an afterburner comprising: an inner flame ring; an outer flame ring, spaced radially outward and concentric to the inner flame ring, which in cooperation with the inner flame ring defines an annular inlet for receipt of hot gas flow from the turbine, together with an annular outlet; an inlet screen between the inner and outer flame rings closely adjacent the annular inlet for metering the flow between the flame rings wherein the turbulence of the wake behind the screen minimizes circumferential temperature gradients of the gas inflow; means for introducing pilot fuel in discrete jets downstream of the inlet screen; circumferentially spaced apart airfoil type swirl vanes radially extending between the inner and outer flame rings downstream of the pilot fuel means; means for igniting the pilot fuel; means for introducing a main flow of fuel outside the flame rings, and means for maintaining the flame rings in fixed relation relative to the engine casing.
 2. The afterburner of claim 1 wherein the attaching means comprise a plurality of retaining links pivotally connected at one end to the engine casing; and a plurality of pistons radially disposed in connection to the flame rings and pivotally connected to the other ends of the retaining links, thereby accommodating thermal expansion of the flame rings.
 3. The afterburner of claim 1 wherein: the inner and outer flame rings are conical frustums cooperatively arranged to define an annular rearwardly diverging passageway.
 4. The afterburner of claim 1 wherein: the inlet screen comprises a plurality of circumferentially spaced cylinders extending radially between the inner and outer flame rings.
 5. The afterburner of claim 1 wherein each swirl vane comprises a generally frusto conical main body portion with an integral tail portion extending rearwardly and circumferentially from the main body portion with the sidewalls of the integral tail portion converging rearwardly from tangential intersection with the frusto conical main body into intersection at the swirl vane trailing edge.
 6. The afterburner of claim 5 wherein each swirl vane includes a sharp integral leading edge portion extending forward of the main body portion with the sidewalls of the integral leading edge portion converging forwardly from tangential intersection with the frusto conical main body into intersection at the swirl vane leading edge and wherein: a sidewall of the integral tail portion extends rearwardly and circumferentially beyond the line of intersection of the converging tail portion sidewalls.
 7. The afterburner of claim 1 wherein: the pilot fuel means includes an inlet conduit terminating between the flame rings at a point closely adjacent the leading edge of a swirl vane and having two spaced apart apertures for directing individual jets of pilot fuel on each side of the adjacent swirl vane, and wherein: the main fuel means includes a plurality of inlet conduits terminating radially outward and inward of both flame rings, each conduit of which includes an aperture for directing a jet of main fuel outside the flame rings.
 8. A flameholder comprising: an inner flame ring; an outer flame ring spaced radially outward and concentric to the inner flame ring which in cooperation with the inner flame ring defines an annular inlet and outlet; an inlet screen between the inner and outer flame rings closely adjacent the annular inlet for metering the flow between the flame rings wherein the turbulence of the wake behind the screen minimizes circumferential temperature gradients of the inflow; means for introducing pilot fuel in discrete jets downstream of the inlet screen; circumferentially spaced apart airfoil swirl vanes radially extending between the inner and outer flame rings downstream of the pilot fuel means; and means for igniting the pilot fuel. 